Dimension 8400 won't start - no lights on power button.

Good evening, all - searched for existing threads - none seem to address this specific issue,

Machine was running normally. I powered it down and opened the case to add new memory - install was quick an uneventful (no sparks, etc :-), but the machine would not restart thereafter.

Analysis so far:

Power button and backside diagnostic lights are all dark.

Motherboard LED lights up with power.

Reinstalling original memory changes nothing (though I'd think that a RAM problem would still allow PC to at least POST).

All capacitors appear okay, with no bulges.

Diagnostic steps so far:

Removed the power cable to mobo and jumped green and ground connectors per other posts - power supply turns on and CPU fan spins to high speed.

Replaced the CMOS battery

Swapped the W1158 power board and cable, and M5989 I/O USB/Audio Panel and cable with similar (though slightly different form factor) 88RXM power and 91NMP I/O ones I had on hand - still no power. This would seem to eliminate the switch path into the mobo, or I am wrong in assumming that - despite identical connectors - the parts are swappable?

I did order a second W1158 button before I got the other 88RXM part to test, so I am at least hopeful that the buttons are different, and another w1158 might work.

In the meantime, does anyone know if or which pins on the USB cable connecting the M5989
I/O panel can be jumped (like the P/S) to simulate the power button
being on?

Any other diagnostic suggestions or ideas would be greatly appreciated.

If you have a 'known' good power supply and are comfortable working around computers, you could try the following:

Remove the power cord, hold power button in for several seconds to discharge the residue power.

Remove all the PCI cards, the video card [if applicable], memory, all peripherals, disconnect the data and power cables to all the drives, check that the front panel cable is connected to motherboard, with nothing else connected to the system, reconnect the power cable and power the system on.

If the fans spin up and there is no change in the diagnostic lights, or beep sequence, it would appear that the motherboard has died and will need to be replaced.

Bev.

If my answer was helpful, please use the 'Did this answer the question' and click: YesForum Member since 2001I am not employed by Dell

I am having the same problem, only mine happend after moving. After setting up at my new home, the power button did nothing. The green AUX_PWR light is lit on the motherboard and none of the diagnostic lights on the back are lit, but the fan does not spin and there are no other indication there is power. Which pins did you determine to be the power button bypass? I was assuming they were pins 23 and 24 based on the circuit board.

Thanks for the link. I tried the power supply jump, and got the fans to spin. I've tried about everything else, but to no avail. Maybe it's time for an upgrade... before I dump too much money into fixing 5 year old technology. That's a sad thought because it's been a good system otherwise.

Well, for better or worse it seems we have the exact same situation - and I was starting to think it was just me (-:

I'm a little more stubborn on letting go, only because I can't believe/accept that the mobo could abruptly fail after working flawlessly for 3+ years, with no short, smoke, popped capacitors, burn marks on the board, etc.

This 2 year old post on the other thread got my attention:

"My 8400, 2yr - 3mo old, suddenly didn't power on at all. No external lights front or back, no sound, no fan, nothing changed when I hit power on. Opening the case there was a small green light on motherboard; all looked OK. I first followed procedures on Dell site and then called. After a bit more than the site tests, the tech support concluded that it was the motherboard and suggested I buy another from them. This is expensive (out of warranty) and I wasn't fully convinced that was the answer. (I've since seen that 3rd parties offer the 'mobo' more cheaply). So I brought the 8400 to a local Geek Squad. They looked at it for several minutes and then pulled and reseated the cpu. They'd seen that loose before on a 775 board. This worked, all rebooted and came up OK. When I got home the 8400 was in the nothing at all status again. I brought it back and this time they also pulled and reseated the CPI boards. Again all OK. I was extremely careful driving home and all was good at home. A few days later (after cleaning lady (a factor or not??)) it was (is) absolutely dead again."

Tried several times to reseat the CPU... no luck. A bypass idea to the power button could at least eliminate that from the mix, but for now it looks like I'm just going to have a Dell paper weight. =0)

I completely disassembled the machine and removed the motherboard from the case. Upon very close inspection, I discovered that 4 of the capacitors that were behind the CPU and under the cooling fan had begun to rupture downward, with some leaking a bit of their contents.

I at least feel vindicated that my memory upgrade attempt didn't cause this (-:, though not realizing the ducting could be easily removed for cleaning (to lower temps) puts the blame back on me.

May try to replace the capacitors for the experience, or just move on to a new machine...

I found a replacement j3492 board for a great price. I bought it, swapped the CPU and cards...and got the same result - no light at power button, no sign of life other than the single green LED on the board itself.

I was certain the capacitors were bad on the old board ("popped" downward), but it would seem that the identical result would eliminate it as the problem. The only variables seemingly left are the power supply and CPU.

As I said before, I jumped the power and ground pins on the power supply per prior suggestions elsewhere and it started - is that a valid enough test to confirm it's okay?

Another question: is it possible the CPU is what died? In other words, will a dead CPU really keep the PC from even powering up?